Fluid dynamic bearing devices are excellent in rotational accuracy and quietness, and hence are suitably used, for example, in a spindle motor for various disk drive devices (such as a magnetic disk drive device for HDDs and an optical disk drive device for CD-ROMs and the like), a polygon scanner motor for laser beam printers (LBPs), or a color-wheel motor for projectors.
For example, the fluid dynamic bearing device disclosed in Patent Literature 1 includes a shaft member including a shaft portion and a flange portion, and a bearing sleeve made of a sintered metal and having an inner periphery along which the shaft portion is inserted. Along with rotation of the shaft member, radial bearing gaps are formed between an outer peripheral surface of the shaft portion and an inner peripheral surface of the bearing sleeve, and a thrust bearing gap is formed between one end surface of the flange portion and one end surface of the bearing sleeve. The inner peripheral surface of the bearing sleeve is provided with radial dynamic pressure generating portions (dynamic pressure generating grooves) for generating a dynamic pressure action in a lubricating oil in the radial bearing gaps, and the one end surface of the bearing sleeve is provided with a thrust dynamic pressure generating portion (dynamic pressure generating grooves) for generating a dynamic pressure action in a lubricating oil in the thrust bearing gap.
Patent Literature 2 discloses a method of forming dynamic pressure generating grooves in the inner peripheral surface of the bearing sleeve made of a sintered metal. In this method, sizing and rotary sizing are performed on a cylindrical sintered metal preform obtained by sintering a green compact of metal powder, and then the dynamic pressure generating grooves are formed in the inner peripheral surface of the sintered metal preform. Specifically, a forming pin provided with a groove pattern for forming the dynamic pressure generating grooves is inserted along an inner periphery of the sintered metal preform, and then the sintered metal preform is held with an upper punch and a lower punch from both sides in an axial direction. In this state, an outer peripheral surface of the sintered metal preform is press-fitted to a die. With this, a compressive force is applied, and the inner peripheral surface is pressed onto the groove pattern and plastically deformed. In this way, the dynamic pressure generating grooves are formed in the inner peripheral surface of the sintered metal preform. Alternatively, when the groove pattern is provided to the upper punch or the lower punch described above, and one end surface of the sintered metal preform is pressed onto the groove pattern, dynamic pressure generating grooves are formed also in the one end surface of the bearing sleeve simultaneously with the formation of the dynamic pressure generating grooves in the inner peripheral surface of the bearing sleeve.
Further, a fluid dynamic bearing device incorporated, for example, in a spindle motor for disk drive devices includes a radial bearing portion for supporting relative rotation of a bearing member and a shaft member in radial directions, and a thrust bearing portion for supporting relative rotation of the bearing member and the shaft member in thrust directions. Of both the bearing portions, the radial bearing portion is generally formed of what is called a fluid dynamic bearing. When the radial bearing portion is formed of the fluid dynamic bearing, a plurality of recessed portions (for example, dynamic pressure generating grooves) for generating a fluid dynamic pressure in a radial bearing gap are provided to an inner peripheral surface of the bearing member or an outer peripheral surface of the shaft member, which face each other across the radial bearing gap. The dynamic pressure generating grooves are generally formed into fine grooves having a groove depth and a groove width of from approximately several micrometers to several tens of micrometers. As a method of forming such fine grooves with high accuracy, the method described, for example, in Patent Literature 4 is publicly known.
Specifically, the method includes: inserting a core rod having an outer peripheral surface provided with a groove-patterned portion in conformity with a shape of dynamic pressure generating grooves along an inner periphery of a cylindrical sintered metal preform to be processed into a bearing member; applying, in this state, a compressive force to the sintered metal preform; causing an inner peripheral surface of the sintered metal preform to bite into the outer peripheral surface of the core rod so as to transfer a shape of the groove-patterned portion to the inner peripheral surface of the sintered metal preform; and then withdrawing the core rod from the inner periphery of the sintered metal preform by utilizing spring-back of the sintered metal preform, which is generated along with release of the compressive force, without deforming the dynamic pressure generating grooves.
However, when the dynamic pressure generating grooves are molded in the inner peripheral surface of the sintered metal preform as described above, a significantly greater compressive force needs to be applied to the sintered metal preform. Thus, a significantly greater force is applied also to the core rod and dies arranged on an outer periphery of the sintered metal preform so as to hold an outer peripheral surface of the sintered metal preform. As a result, the core rod and the dies are liable to be subjected to abrasion and the like, and hence the dies need to be frequently replaced, which causes an increase in cost of forming the dynamic pressure generating grooves, by extension, manufacturing cost of the fluid dynamic bearing device. As a countermeasure, as means for reducing the cost of forming the dynamic pressure generating grooves, attention has been drawn again to the formation of the dynamic pressure generating grooves in the outer peripheral surface of the shaft member.
The shaft member is generally made of a metal material excellent in strength and rigidity, such as quenched stainless steel. As a method of forming the plurality of dynamic pressure generating grooves in the outer peripheral surface of the shaft member made of such metals, there may be employed trimming, etching, rolling, or the like. Of those, the rolling tends to be used in many cases because dynamic pressure generating grooves of high accuracy can be formed relatively easily at low cost. For example, Patent Literature 5 describes a specific procedure generally employed for forming, by rolling, the dynamic pressure generating grooves in the outer peripheral surface of the shaft member. Specifically, the procedure includes: pressing rolling dies onto a shaft preform finished to have a predetermined axial diameter so as to form dynamic pressure generating grooves in an outer peripheral surface of the shaft preform; performing heat treatment on the shaft preform so as to obtain a quenched shaft; and performing last finishing such as grinding on the outer peripheral surface of the quenched shaft provided with the dynamic pressure generating grooves, to thereby obtain a shaft member as a finished product having an outer peripheral surface finished to have predetermined accuracy together with the dynamic pressure generating grooves and hill portions that define the dynamic pressure generating grooves.
Further, when the thrust bearing portion is formed of what is called a dynamic pressure bearing, a flanged shaft member including a shaft portion and a flange portion is normally used as the shaft member. In this case, the radial bearing gap of the radial bearing portion is formed between an outer peripheral surface of the shaft portion and a surface opposed thereto, and the thrust bearing gap of the thrust bearing portion is formed between an end surface of the flange portion and a surface opposed thereto.
As the flanged shaft member, there are used an integrated type in which the shaft portion and the flange portion are formed integrally with each other by a machining process such as the trimming, and a separate type in which the shaft portion and the flange portion are produced separately from each other and then integrated with each other by appropriate means. The integrated type flanged shaft member is advantageous in that accuracy between the shaft portion and the flange portion (such as perpendicularity) can be easily enhanced, and hence high rotational accuracy of the fluid dynamic bearing device can be secured. However, in addition to requirement of dedicated processing equipment, material loss is significant, and hence markedly high production cost is required. Meanwhile, the separate type flanged shaft member is advantageous in that properties required for each of the shaft portion and the flange portion can be easily satisfied, and in addition, the separate type flanged shaft member can be mass-produced at cost lower than that of the integrated type flanged shaft member.
In particular, as in the case of the flanged shaft member described in Patent Literature 6, when a thrust dynamic pressure generating portion (in which dynamic pressure generating grooves for generating a fluid dynamic pressure in a thrust bearing gap are arrayed in a herringbone pattern and the like) is molded in each end surface of the flange portion through a pressing process, and simultaneously, the flange portion is fixed to one end of the shaft portion, a manufacturing step of the shaft member is simplified, and another thrust dynamic pressure generating portion need not be provided to an end surface of a member, which faces the end surface of the flange portion across the thrust bearing gap. Thus, a manufacturing step of the fluid dynamic bearing device can be simplified, and hence manufacturing cost of the fluid dynamic bearing device can be reduced.